1. Field of the Invention
The invention relates generally to devices for aligning and coupling optical fibers. More specifically, the invention relates to a method and an apparatus for making a capillary splice.
2. Background Art
Fiber-optic systems require means for transferring energy from one optical fiber to another without undue power loss. One method for transferring energy from one optical fiber to another involves positioning the optical fibers in an end-to-end relationship so that light emanating from one fiber end face is directed into the adjacent fiber end face.
Capillary splices can be used to align sets of optical fibers in an end-to-end relationship. FIG. 1 shows a cross-section of a capillary splice 1, which includes a capillary tube 2 having a longitudinal bore 3. Terminal ends of optical fibers 4, 5 are inserted into the bore 3 to place the optical fibers 4, 5 in an end-to-end relationship. It is usually desirable to make the diameter of the bore 3 only a few microns larger than the diameter of the optical fibers 4, 5 so that the optical axes of the optical fibers 4, 5 remain substantially aligned when inserted into the bore 3. Funnel-like apertures 6, 7 are formed at the ends of the bore 3 to facilitate insertion of the optical fibers 4, 5 into the bore 3. The funnel-like apertures 6, 7 also provide a mounting location for bonding material 8, 9, which is used to secure the optical fibers 4, 5 to the capillary tube 2. In general, one or both ends of the bore 3 may be terminated with a funnel-like aperture.
It is important that the capillary splice is manufactured with extreme precision to avoid undue power loss when coupling light between the optical fibers. U.S. Pat. No. 4,822,389, issued to Berkey, discloses a method for making a capillary splice. The method involves placing the bore of a capillary glass tube under pressure by filling the bore with a fluid, such as air, nitrogen, and the like, and then applying localized heat to the capillary glass tube. As the glass tube is heated to its softening point, the inside wall of the bore starts to expand within the softened area by the pressure of the fluid within the bore. The pressure of the fluid causes a bubble to form within the glass tube.
FIG. 2A shows a bubble 10a forming within a glass tube 11 as the bore 12 of the glass tube 11 is pressurized with fluid 13 and the glass tube 11 is heated locally to its softening point by a burner 14. The glass tube 11 is rotated as it is heated to allow for uniform heat distribution across the glass tube 11. As the glass tube 11 is rotated and subjected to localized heating, the bubble 10a continues to expand until it occupies a major portion of the diameter of the glass tube 11. FIG. 2B shows the bubble 10a occupying a major portion of the diameter of the glass tube 11. After forming the bubble 10a, the glass tube 11 may be stretched (or pulled) along its axial axis to size the bubble 10a to a desired length and diameter. FIG. 2C shows another bubble 10b formed within the glass tube 11 using the process just described. The bubble 10b is spaced a desired distance from the bubble 10a. To form the capillary splice, the glass tube 11 is scored along its exterior surface at about the center of each of the bubbles 10a, 10b. Thereafter, the glass tube 11 is severed along the score lines to produce the capillary splice.
Forming a capillary splice that meets desired specifications requires careful control and coordination of the amount of pressure inside the glass tube, the amount of heat applied to the glass tube, where the heat is applied to the glass tube, the orientation of the glass tube while the bubble is being formed, and the amount of pull applied to the glass tube after forming the bubble. The amount of pull applied to the glass tube would depend on the size of the bubble achieved after applying pressure and heat to the glass tube and the desired length and diameter of the bubble. Controlling and coordinating these process parameters manually to achieve bubbles that meet desired specifications is very difficult. Measurements would have to be made after each bubble is formed to determine needed process parameters. In some cases, these measurements would have to be performed manually, with the results varying from one operator to another.